Externally Guided and Directed Field Induction Resistivity Tool

ABSTRACT

In one aspect of the invention, an induction resistivity tool incorporated into a downhole tool string comprises an outer wall of a downhole component comprising an outer diameter and at least one induction transmitter assembly disposed along the outer diameter. The at least one transmitter assembly comprises at least one induction transmitter coil wound about at least one core. The at least one transmitter coil is adapted to project an induction signal outward from the outer wall when the at least one transmitter coil is carrying an electrical current. The transmitter assembly is adapted to create electromagnetic fields that originate the induction signal from outside the outer wall and substantially prevent the signal from entering the outer wall.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/776,447 filed on Jul. 11, 1007 and entitled Externally Guided andDirected Field Induction Resistivity Tool. U.S. patent application Ser.No. 11/776,447 also claimed priority to Provisional U.S. PatentApplication No. 60/914,619 filed on Apr. 27, 2007 and entitledResistivity Tool. All of the above mentioned references are hereinincorporated by reference for all that they contain.

BACKGROUND OF THE INVENTION

For the past several decades, engineers have worked to develop apparatusand methods to effectively obtain information about downhole formations,especially during the process of drilling and following this processusing wireline methods or pushed tool methods for use in horizontalwells. All of these methods are collectively referred to in the industryas logging. During the drilling process and, with time afterward,drilling fluids begin to flush and intermingle with the natural fluidsin the formation forming an invasion zone near the drilled borehole.This fluid exchange increases with time and the formation wall candegrade or become damaged with further drilling operations which canmask or alter information about the formation that is of interest.Logging-while-drilling (LWD refers to a set of processes commonly usedby the industry to obtain information about a formation near the drillbit during the drilling process in order to transmit the informationfrom components located downhole on oil and gas drilling strings to theground's surface. Measurement-while-drilling (MWD) refers to a method ofLWD that will store part and transmit the remaining information to thesurface or store all of the information collected during drilling forlater retrieval and download into surface electronics. LWD methods arealso used in smart drilling systems to aid or direct the drillingoperations and in some cases to maintain the drill in a specific zone ofinterest. The terms MWD and LWD are often used interchangeably in theindustry and LWD will be used here to refer to both methods with theunderstanding that the LWD encompasses systems that collect formation,angular rotation rate and depth information and store this informationfor later retrieval and/or transmission of this information to thesurface while drilling.

A common sensor used in logging systems is for the measurement ofresistivity or the complement conductivity. The resistivity of theformation is quite often measured at different depths into the formationto determine the amount of fluid invasion and aid in the calculation oftrue formation resistivity. The formation resistivity is generally usedwith other sensors in an analysis to determine many other formationparameters. There are various types of resistivity sensors includingdirect current (DC), and alternating current (AC) focused resistivitywhich utilizes one or more electrodes devices, AC scanned resistivitywhich measures in a specific circumferential or angular pattern aroundthe borehole and a fourth type called induction or propagationresistivity which also utilizes AC methods. Induction resistivitysensors generally use lower frequencies below 100 KHz while propagationsensors use higher frequencies. The terms induction sensor or inductiontool will be used interchangeably here and will refer to both inductionand propagation resistivity methods.

Induction tools with varying number and combinations of transmitter(s)and receiver(s) with varying separation distances and operatingfrequencies have been used to explore formations at various depths ofinvestigation.

The prior art comprises the following references to resistivity toolsand resistivity logging which have a common design problem that eitherallows the transmitted field to penetrate the induction tool or if ashield is utilized, do not actively direct the field away fromelectrical and magnetic tool surfaces and materials.

U.S. Pat. No. 6,677,756 to Fanini, et al, which is herein incorporatedby reference for all that it contains, discloses an induction tool forformation resistivity evaluations. The tool provides electromagnetictransmitters and sensors suitable for transmitting and receivingmagnetic fields in radial directions.

U.S. Pat. No. 6,359,438 to Bittar, which is herein incorporated byreference for all that it contains, discloses a resistivity tool for usein an LWD system that includes a transmitter array with multipletransmitters positioned above a pair of receivers. The transmitters areselectively energized, causing current to be induced in the collar ofthe tool.

U.S. Pat. No. 6,577,129 to Thompson, et al, which is herein incorporatedby reference for all that it contains, discloses an electromagnetic wavepropagation resistivity borehole logging system comprising multiplegroups of electromagnetic transmitter-receiver arrays operating at threefrequencies.

U.S. Pat. No. 6,538,447 to Bittar, which is herein incorporated byreference for all that it contains, discloses a multi-mode resistivitytool for use in a logging-while-drilling system that includes anasymmetric transmitter design with multiple transmitters capable ofgenerating electromagnetic signals at multiple depths of investigation.

U.S. Pat. No. 7,141,981 to Folbert, et al, which is herein incorporatedby reference for all that it contains, discloses a resistivity loggingtool suitable for downhole use that includes a transmitter, and twospaced apart receivers. The resistivities at the two receivers arecorrected based on measuring the responses of the receivers to acalibration signal.

U.S. Pat. No. 6,218,842 to Bittar, et al, which is herein incorporatedby reference for all that it contains, discloses a resistivity tool foruse in LWD systems that includes an asymmetric transmitter design withmultiple transmitters capable of generating EM signals at multiplefrequencies.

U.S. Pat. No. 5,045,795 to Gianzero, et al, which is herein incorporatedby reference for all that it contains, discloses a coil array which isinstalled on a MWD drill collar for use in a resistivity logging system.The drill collar is provided with upper and lower coil support rings.These are toroids which support individual coil segments, and areconnected by suitable magnetic shorting bars. The coil segments andshorting bars inscribe a specified solid angle or azimuthal extent.

U.S. Pat. No. 5,606,260 to Giordano, et al, which is herein incorporatedby reference for all that it contains, discloses a microdevice isprovided for measuring the electromagnetic characteristics of a mediumin a borehole. The microdevice includes at least one emitting ortransmitting coil (31), and at least one receiving coil (41,51). Themicrodevice generates an A.C. voltage at the terminals of thetransmitting coil and measures a signal at the terminals of thereceiving coil. The microdevice also includes an E-shaped electricallyinsulating, soft magnetic material circuit serving as a support for eachof the coils and which is positioned adjacent to the medium in theborehole.

U.S. Pat. No. 6,100,696 to Sinclair, which is herein incorporated byreference for all that it contains, discloses a directional inductionlogging tool is provided for measurement while drilling. This tool ispreferably placed in a side pocket of a drill collar, and it comprisestransmitter and receiver coils and an electromagnetic reflector.

U.S. Pat. No. 6,163,155 to Bittar, et al, which is herein incorporatedby reference for all that it contains, discloses a downhole method andapparatus for simultaneously determining the horizontal resistivity,vertical resistivity, and relative dip angle for anisotropic earthformations.

U.S. Pat. No. 6,476,609 to Bittar, et al, which is herein incorporatedby reference for all that it contains, discloses an antennaconfiguration in which a transmitter antenna and a receiver antenna areoriented in non-parallel planes such that the vertical resistivity andthe relative dip angle are decoupled.

U.S. patent application Ser. No. 11/676,494 to Hall et al., which isherein incorporated by reference for all that it contains, discloses aninduction resistivity tool comprising a flexible ring of magneticallyconducting material disposed intermediate an induction coil and asurface of an annular recess.

U.S. patent application Ser. No. 11/687,891 to Hall et al., which isherein incorporated by reference for all that it contains, discloses aresistivity tool comprising an actuator in a downhole component that isadapted to put an electrically conductive element into and out ofelectrical contact with at least one electrically insulated inductioncoil and thereby change an optimal signal frequency of the at least onecoil.

FIELD OF THE INVENTION

The present invention relates to a measurement procedure used to assessand aid in the recovery of petroleum, gas, geothermal and other mineralsand resources. And more particularly, this invention relates to thefield of induction resistivity tools for tool strings employed in suchexploration. The present invention generally relates to a well loggingtool with one or more transmitters and one or more receivers thatmeasure the resistivity and other formation parameters adjacent to thewellbore. More particularly, the present invention relates to a methodof generating, directing and shielding a field adjacent to and away froman electrically conductive structure with a minimum of interference fromthe electrical or magnetic properties of the structure that would alter,distort or minimize the generated field. The invention applies toMWD/LWD tools, pad on arm based tools and pushed tools for use invertical to horizontal wellbores.

BRIEF SUMMARY OF THE INVENTION

The embodiment of the invention is a configuration of windings, coils,spirals or antenna that generates, and actively guides and directs afield in a path external to and away from an electrically conductivestructure with some electrical and/or magnetic properties.

In a padded, pushed or LWD induction resistivity tool, the preferredembodiment of a single transmitter element is a modified, alternatingcurrent (AC) driven Halbach array formed from four or more windings,coils, spirals or antenna that generates an augmented fieldperpendicular to the long axis of the metal drill collar or mandrel suchthat the field can be guided and directed away from the collar ormandrel. The transmitter element may include one or more windings, coil,spiral or antenna, herein referred to as bucking coils, positioned insuch a manner as to aid in directing the field away from the collar ormandrel. The transmitter element may utilize insulating, electrical,ferrite and/or magnetic materials to guide the field to minimize theinfluence of the metal collar or mandrel. A single or plurality oftransmitter elements are placed either partially or completelycircumferentially around the perimeter of the collar or mandrel at anyangle to form a single transmitter. In padded embodiments, one or moretransmitter elements may be utilized. Completely or partiallycircumferential or padded implementations of the transmitter elementallow the system to preferentially measure only a small angular area ofthe formation for detailed analysis or smart tool or guided drillingapplications.

In a padded, pushed or LWD induction resistivity tool, the preferredembodiment of a single receiver element is one or more winding, coil,spiral or antenna that detects the generated field with a minimum ofinterference from the collar or mandrel. The receiver element mayutilize insulating, electrical, ferrite and/or magnetic materials toguide and direct the field to minimize the influence of the collar ormandrel. A single or plurality of receiver elements are placed eitherpartially or completely circumferentially around the perimeter of thecollar or mandrel at any angle to form a single receiver. In paddedembodiments, one or more receiver elements may be utilized and orientedat any angle. Circumferentially or padded implementations of thereceiver element allow the system to preferentially measure only a smallangular area of the formation for detailed analysis or smart tool orguided drilling applications.

Any number of transmitters and receivers can be used to obtain a givendepth of investigation into the formation and a given vertical field orbed resolution.

The plurality of transmitter, receiver and bucking windings, coils,spirals or antenna may be electrically connected in parallel or inseries. One or more of the plurality of transmitters may be adapted toswitch between a series and parallel connection with another of theplurality of transmitters.

The induction transmitter assembly may be disposed within one or moreradial recesses disposed in the outer diameter of the outer wall of thecollar or mandrel. The recesses may be horizontal, perpendicular to thelong axis of the collar or mandrel or at any angle.

The resistivity tool may be in communication with a downhole network.The resistivity tool may be incorporated into a bottom hole toolassembly. The at least one induction transmitter assembly may be tiltedwith respect to an axis of the downhole tool string.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of an embodiment of a downhole toolstring.

FIG. 2 is a perspective diagram of an embodiment of an inductiveresistivity tool.

FIG. 3 is a cross-sectional diagram of an embodiment of an inductiontransmitter assembly in an inductive resistivity tool.

FIG. 4 is a perspective diagram of an embodiment of an inductiontransmitter assembly disposed in a radial recess with a detailedenlargement.

FIG. 5 is a perspective diagram of another embodiment of an inductiontransmitter assembly disposed on a padded arm on a downhole tool.

FIG. 6 is a perspective diagram of another embodiment of an inductiontransmitter assembly disposed in a radial recess used for angular radialinvestigation.

FIG. 7 is a cross-sectional diagram of an embodiment of an inductionreceiver assembly disposed in a radial recess.

FIG. 8 is a perspective diagram of an embodiment of an inductionreceiver assembly disposed in a radial recesses with a detailedenlargement.

FIG. 9 is a perspective diagram of another embodiment of the inductiontransmitter assembly disposed in a radial recess.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a downhole tool string 31 may be suspended by aderrick 32. The tool string may comprise one or more downhole components36, linked together in a tool string 31 and in communication withsurface equipment 33 through a downhole network. Having a network in thetool string 31 may enable high-speed communication between each deviceconnected to it and facilitate the transmission and receipt of databetween sensors, energy sources, and energy receivers. The tool string31 is depicted in a vertical drilled hole but it may be at any angleincluding horizontal.

The tool string 31 or surface equipment 33 may comprise an energy sourceor multiple energy sources. The energy source may transmit electricalcurrent to one or more downhole components 36 on the bottom holeassembly 37 or along the tool string 31. In some embodiments of theinvention, one or more downhole component 36 may comprise sensors. Thesesensors may sense gamma rays, radioactive energy, resistivity, torque,pressure, or other drilling dynamics measurements or combinationsthereof from the formation being drilled. Any combination of downholecomponents 36 in a tool string 31 may be compatible with the presentinvention.

Data may be transmitted along the tool string 31 through techniquesknown in the art. A preferred method of downhole data transmission usinginductive couplers disposed in tool joints is disclosed in the U.S. Pat.No. 6,670,880 to Hall, et al, which is herein incorporated by referencefor all it discloses. An alternate data transmission path may comprisedirect electrical contacts in tool joints such as in the systemdisclosed in U.S. Pat. No. 6,688,396 to Floerke, et al., which is hereinincorporated by reference for all that it discloses. Another datatransmission system that may also be adapted for use with the presentinvention is disclosed in U.S. Pat. No. 6,641,434 to Boyle, et al.,which is also herein incorporated by reference for all that itdiscloses. In some embodiments, of the present invention alternativeforms of telemetry may be used to communicate with the downholecomponents 36, such as telemetry systems that communicate through thedrilling mud or through the earth. Such telemetry systems may useelectromagnetic or acoustic waves. The alternative forms of telemetrymay be the primary telemetry system for communication with the toolstring 31 or they may be back-up systems designed to maintain somecommunication if the primary telemetry system fails. A data swivel 34 ora wireless top-hole data connection may facilitate the transfer of databetween components 36 of the rotatable tool string 31 and the stationarysurface equipment, such as a control module 33.

Preferably the downhole tool string 31 is a drill string. In otherembodiments the downhole tool string 31 is part of a coiled tubinglogging system or part of a production well. In the present embodiment,an embodiment of a resistivity tool 201 in accordance with the presentinvention is shown producing a transmitter field 30 and projecting thetransmitter field 30 through the formation 40.

Control equipment may be in communication with the downhole tool stringcomponents 36 through an electrically conductive medium. For example, acoaxial cable, wire, twisted pair of wires or combinations thereof maytravel from the surface to at least one downhole tool string component.The medium may be in inductive or electrical communication with eachother through couplers positioned so as to allow signal transmissionacross the connection of the downhole component and the tool string. Thecouplers may be disposed within recesses in either a primary orsecondary shoulder of the connection or they may be disposed withininserts positioned within the bores of the drill bit assembly and thedownhole tool string component. As the control equipment receivesinformation indicating specific formation qualities, the controlequipment may then change drilling parameters according to the datareceived to optimize drilling efficiency. Operation of the drill string31 may include the ability to steer the direction of drilling based onthe data either manually or automatically.

Referring now to FIG. 2 an embodiment of an inductive resistivity tool201 is shown as part of a downhole tool string 31 which may bestationary or rotating. The resistivity tool 201 is shown intermediatefirst and second tool joints 202, 203. A transmitter field 207 is shownbeing produced by two transmitters 204, and being received by one ofthree receivers 205. The transmitter field 207 induces anelectromagnetic field into the formation, which then in turn induces thereceivers 205. By projecting the transmitter field through the formationand comparing the amplitude and phase of the received signal to that ofthe transmitted signal, the resistivity and other parameters of theformation may be determined. Because hydrocarbon and/or petroleumproducts in the formation are typically non-conductive, resistivitymeasurements may be used to determine the petroleum potential of aformation during the drilling process. The preferential projection ofthe transmitter field 207 away from the tool string 31 by thetransmitter may allow the wall 301 of the downhole component 36 tocomprise a magnetically and electrically conductive material. A singletransmitter 204 or plurality of transmitters 204 may be disposed on asingle tool 201. In embodiments comprising a plurality of transmitters204, each transmitter 204 may be selectively energized. A sleeve 206 maybe disposed around the components of the resistivity tool 201 to protectthem from mud and/or debris. Although specific numbers of receivers 205and transmitters 204 have been shown in the present embodiment, anycombination of any number of receivers and transmitters 205, 204 may beconsistent with the present invention. In some embodiments the tool 201may be incorporated into a drilling string, a tool string, a pushed coiltubing string, a wireline system, a cable system, or combinationsthereof.

Referring now to FIG. 3, a cross sectional view of an embodiment of aportion of a resistivity tool 201 is shown without a protective sleeve206 and disposed in a downhole component 36. The downhole component 36has an outer wall 301 surrounding a central bore 302 through whichdrilling mud (not shown) may be transferred. The outer wall 301comprises an annular radial recess 303 formed in its outer diameter 304.An induction transmitter assembly 305 is disposed within the radialrecess 303. In this embodiment, the transmitter assembly 305 comprises aplurality of electromagnetic induction transmitter coils 306 which mayalso have adjacent directing bucking coils 313.

The transmitter coil 306 may be wound about at least one core 307. Thetransmitter coils 306 are arranged in an orientation to create an array315. The array 315 may be a Halbach array 316 or a modified version ofsuch an array. For the purposes herein, a modified Halbach array willalso be referred to as a Halbach array. A Halbach array 316 creates anaugmented field outward toward the formation 40 and away from the toolwhile simultaneously forming a canceled field between the coils and thetool 201.

The Halbach Array 316 may be modified with different coil widths andcoil sizes in order to preferentially project a field in only onedirection away from the tool. The Halbach Array 316 may also be modifiedwith compound coils (not shown) in which one or more of the coils form acompound coil with two or more windings to preferentially project afield in only one direction away from the tool. The compound coils maybe at any angle or orientation with each other. The transmitter assembly305 may be mounted in any orientation and at any angle. It is furtherunderstood that the use of the term “coils” herein may be coils withindividual windings or integral windings as part of the core 307, aspiral, or the coil may be an antenna. In embodiments of the inventionwhere the coils 306 comprise individual or integral windings, atransmitter coil 306 may comprise between 1 and 1000 coil turns. Eachcore 307 may be comprised of a magnetically conductive material, such asferrite. At least one core 307 may comprise an electrically insulatingcylinder that is disposed around a dielectric material. The transmittercoils 306 may each comprise any number of coil turns, spirals or otherelectrical pattern. In some embodiments of the invention, thetransmitter assembly 305 may comprise a single transmitter coil 306 thatis wound about a plurality of cores 307.

When an electrical alternating current (AC) flows through thetransmitter coil 306, an induction signal is transmitted and thendirected away from the transmitter coil 306 by the action of the Halbachaugmentation effect and may be aided by bucking coil(s) 313. Thecanceled field below the transmitter may allow for the use of amagnetically conductive material in the outer wall 301 withoutinterfering with the action of the resistivity tool 201. Electricalcurrent may be supplied to the transmitter assembly 305 via anelectrically conductive medium 311. Electrically conductive medium 311may comprise a plurality of copper wires 312, coaxial cable, twistedpairs of wire, or combinations thereof which may extend fromelectrically conductive medium 311 to locations throughout thetransmitter assembly 305.

Referring now to FIG. 4, a perspective diagram of a resistivity tool 201discloses an enlarged view of an embodiment of a transmitter assembly305. The transmitter assembly 305 comprises a plurality of transmittercoils 306 arranged in a Halbach Array 316 and bucking coils 313 disposedalong the outer diameter 304 and within a radial recess 303. In oneembodiment the transmitter assembly 305 is circumferentially spans theouter diameter 304 of the tool 201. Each of the plurality of thetransmitter coils 306 and bucking coils 313 may be wound about at leastone core 307. The transmitter assembly 305 is separated from the bottomof annular radial recess by a shield 404 which may be an insulatorand/or magnetically conductive material such as ferrite. Themagnetically conductive electrically insulating material may compriseferrite fibers, shavings, powder, crystals, or combinations thereof.

FIG. 5 discloses the invention embodied as a padded inductiveresistivity tool 504. A transmitter assembly 305 and a receiver assembly501 are each mounted on an outer extendable pad 507 connected to theouter wall 301 by an extendable arm assembly 506. The transmitterassembly 305 is adapted to direct transmitter field 207 away from thepad 507 and into a selected portion of the formation.40. The extendablearm assembly 506 may allow the transmitter assembly 305 to be disposedaway from the outer wall 301 of tool 504 and proximate the formation 40.FIG. 5 also discloses a plurality of receiver coils 505 disposed on pad507. Each receiver coil 505 is wound about at least one core 307. Thisembodiment makes further use of the Halbach augmented field side of thearray by pointing it away from the pad 507, arm 506 and tool 504 suchthat these metal structures do not influence the generated field.Further the augmented side of the Halbach Array 316 directs the field 30away from the pad 507 and into a selected portion of the formation.40adjacent to the pad 507. The receiver assembly 501 is depicted withthree different receiver coils 505 which are shown oriented in threeorthogonal directions (x, y and z). The receiver assembly may onlyembody a single coil or a plurality of coils mounted in any direction.This embodiment would normally be used in pushed applications such ascoiled tubing logging or in open hole wireline logging applications.

Referring now to FIG. 6, a plurality of transmitter and receiversassemblies 305, 501 may be mounted in small radial recesses 602 in theouter wall 301 of a resistivity tool. The transmitter and receiverassemblies 305, 501 may be disposed circumferentially around the outerdiameter 304 of the tool 201. The specific location of each transmitterand receiver assembly 305, 501 may direct the field 30 into a selectedportion of the formation 40. As the downhole components 36 and theinduction resistivity tool 201 rotate axially as indicated by arrow 208,the directed field may sweep through a continuous path of selectedportions of the formation 40. The induction resistivity tool 201 mayconsist of a single or plurality of Halbach transmitter assemblies 305and receiver assemblies 501. The same measurement could be achieved withthe embodiment depicted in FIGS. 3 and 4 or in FIG. 9 by selectivelyusing sections of the transmitter 204 and receiver 205.

Open space in annular recesses 303, 602 around transmitter and receiverassemblies 305, 501 may be filled with a potting material and/or coveredwith a protective sleeve 206. The potting material may comprise amaterial selected from the group consisting of polymers, organicmaterials, thermoset polymers, vinyl, an aerogel composite, a syntheticbinder, thermoplastic polymers, an epoxy, natural rubber, fiberglass,carbon fiber composite, polyurethane, silicon, a fluorinated polymer,grease, polytetrafluoroethylene, a perfluroroalkoxy compound, resin,soft iron, ferrite, a nickel alloy, a silicon iron alloy, a cobalt ironalloy, a mu-metal, a laminated mu-metal, barium, strontium, carbonate,samarium, cobalt, neodymium, boron, a metal oxide, ceramics, cermets,ceramic composites, rare earth metals, and combinations thereof.

Formations 40 may comprise varying resistivity characteristics dependingon their composition. These characteristics may require the use ofdifferent voltages or frequencies to obtain logging information. Changesin voltage or frequency may be facilitated by the ability to changeadjacent coils 306 between parallel and series connections. In someembodiments of the invention a signal alteration component (not shown)such as a voltage controlled oscillator (VCO) may be disposed between apower source and the transmitter assembly 305.

FIG. 7 is a cross-sectional view of the induction tool 201 depictinganother embodiment of a receiver assembly 501. The receiver assembly 501may comprise longitudinal receiver coils 704, vertical receiver coils705, horizontal receiver coils 706, circumferentially wrapped receivercoils, or combinations thereof.

Referring now to FIG. 8, a perspective diagram of a resistivity tool 201discloses an enlarged view of an embodiment of a receiver assembly 501in which a receiver induction coil 505 is wound about a plurality ofcores 307. A shield 404 is disposed intermediate the receiver assembly501 and a outer surface of the outer wall 301 disposed in the annularrecess 303. The shield 404 may be comprised of an insulator and/orferrite material. The coils 505 may have a core 307 comprising air,ferrite or another material.

Each of the induction coils 306, 505 may be electrically parallel to oneanother. In some embodiments of the invention some of the inductioncoils 306, 505 may be electrically connected in series. Parallelconnections of induction coils 306, 505 may be advantageous inembodiments where an inductance of the induction coil 306, 505 wouldotherwise be so great that it would require a prohibitive amount ofvoltage or amperage to induce a transmitter field 207 of the desiredstrength. In some embodiments, a plurality of the cores 307 may be wiredtogether forming an induction segment 801. These segments 801 may beselectively turned on and off to aid in taking complex measurements. Asdisclosed in FIG. 8, a first induction coil 802 may be adapted to switchbetween a series and parallel connection with a second induction coil803. This adaptation may be accomplished by connecting the first andsecond coils 802, 803 via a connector switch 804.

FIG. 9 discloses an embodiment of the invention in which a plurality ofHalbach Arrays 316 is mounted in a radial recess 303. Each of theplurality of Hallbach Arrays 316 is disposed parallel to the longcentral axis of the tool's central bore 302. This embodiment may have asingle or plurality of transmitter assemblies 305 mountedcircumferentially around the tool 201. This embodiment may use the sameor different size and shape coils and may also make use of compoundcoils to form preferential field orientations around the tool perimeteror along a long central axis 910 of the tool 201 while directing theaugmented side of the generated field out and away from the tool 201.This embodiment may also use a shield 404.

Whereas the present invention has been described in particular relationto the drawings attached hereto, it should be understood that other andfurther modifications apart from those shown or suggested herein, may bemade within the scope and spirit of the present invention.

1. An induction resistivity tool incorporated into a downhole toolstring, comprising: an outer wall of a downhole component comprising anouter diameter; at least one induction transmitter assembly disposedalong the outer diameter; the at least one transmitter assemblycomprising at least one induction transmitter coil wound about at leastone core; the at least one transmitter coil projects an induction signaloutward from the outer wall when the at least one transmitter coil iscarrying an electrical current; and wherein transmitter assembly createselectromagnetic fields that originate the induction signal from outsidethe outer wall and substantially prevent the signal from entering theouter wall.
 2. The tool of claim 1, wherein the at least one transmitterassembly comprises a plurality of induction transmitter coils, each coilbeing wound about at least one core.
 3. The tool of claim 2, wherein atleast one of the plurality of induction transmitter coils comprisesfirst and second ends and at least one bucking coil disposed adjacenteach of the first and second ends and the outer wall.
 4. The tool ofclaim 1, wherein at least one induction coil is adapted to switchbetween a series and parallel connection with at least one otherinduction coil.
 5. The tool of claim 2, wherein at least one core spansat least one third of a total circumference along the outer diameter. 6.The tool of claim 5, wherein a transmitter coil is wound about first andsecond ends of the core and the ends are each disposed orthogonally tothe outer wall.
 7. The tool of claim 1, wherein the transmitter assemblyis circumferentially spans the outer diameter.
 8. The tool of claim 1,wherein a plurality of transmitter assemblies is disposedcircumferentially around the outer diameter.
 9. The tool of claim 1,wherein a shield of magnetically conductive and electrically insulatingmaterial is disposed intermediate the at least one transmitter coil andthe outer wall.
 10. The tool of claim 9, wherein the magneticallyconductive and electrically insulating material comprises ferritefibers, shavings, powder, crystals, or combinations thereof.
 11. Thetool of claim 1, wherein the at least one induction transmitter coil iswound about a plurality of cores.
 12. The tool of claim 1, wherein thecoil comprises between 1 and 1000 coil turns.
 13. The tool of claim 1,wherein the tool is incorporated into a drilling string, a tool string,a pushed coil tubing string, a wireline system, a cable system, orcombinations thereof.
 14. The tool of claim 1, wherein the at least onetransmitter is disposed on an outer extendable pad that extends awayfrom the outer wall and toward the formation and is connected to theouter wall by an arm assembly.
 15. The tool of claim 1, wherein at leastone core comprises a magnetically conductive material.
 16. The tool ofclaim 1, wherein the at least one core comprises an electricallyinsulating cylinder disposed around a dielectric material.
 17. The toolof claim 1, wherein the at least one induction transmitter assembly istilted with respect to the downhole tool string.
 18. The tool of claim1, wherein the outer wall of the downhole component comprises amagnetically conductive material.
 19. The tool of claim 1, wherein thetool comprises at least one induction receiver assembly comprising atleast one receiver coil wound about at least one core.
 20. The tool ofclaim 19, wherein the at least one receiver assembly compriseslongitudinal receiver coils, vertical receiver coils, horizontalreceiver coils, circumferentially wrapped receiver coils, orcombinations thereof.